1. Field of the Invention
The present invention concerns a device for signalling the position of a mobile member.
The invention finds an application in electrical engineering and this specification uses an illustrative example which is not of a limiting nature, of course.
The example concerns the signalling of the position of the contacts of an electrical switch such as a circuit-breaker. It is essential for the operator of an electrical plant incorporating devices such as circuit-breakers to be certain of the open or closed condition of the contacts of each circuit-breaker; this information, usually available at each circuit-breaker, is centralized at a control and monitoring station; it is essential that any failure of the signalling link connecting each of the devices to said station be reported immediately, failing which the signal received at the station may give reason to believe that a device is in a given state whereas it is in fact in the opposite state: any such error can have unfortunate consequences for the operating authority.
For the same reasons it is essential that the device which performs the signalling should, as far as possible, indicate that it is faulty itself or that its power supply has failed. This self-monitoring makes it possible to increase significantly the availability of the self-monitored part of the device.
Of course, the problem is not limited to sensing the position of circuit-breaker contacts; in electrical plant the state of pressure-switches, the oil pressure in hydraulic control circuits, oil levels, etc may all need to be signalled by means of a signalling contact.
Approximately 30% of serious failures of electrical plants are due to a bad signal contact; this indicates the magnitude of the problem.
An object of the invention is therefore to provide a device for signalling the state of a device by sensing this state and transmitting corresponding information, which guarantees error-free operation in respect of the sensed state, and which signals immediately its own failure and failure of the signalling link.
Another object of the invention is to provide a device which is insensitive to external influences such as electrical or magnetic fields and common mode interference when links without galvanic isolation are employed.
It is well known that the use of opto-electronic devices, fiber optics and screening provide a solution to the last-mentioned requirement. The next problem is that of the consumption of the device; another object of the invention is therefore to provide a device requiring for its operation no more energy than that which is available from a photovoltaic cell.
2. Description of the Prior Art
The U.S. Pat. No. 4,626,621 describes a circuit for determining the position of an object comprising two LR circuits driven by a squarewave signal from a pulse generator. One of the LR circuits includes a fixed inductor. The other includes an inductor whose value varies with the position of the object. Voltages are established in the circuits from a time t1 according to different exponential laws in the two circuits and the respective times t2 and t3 to establish a voltage of given value Vo in each of the two circuits is measured. The ratio (t3-t1)/(t2-t1) provides a value representing the position concerned.
A circuit of this kind is complex because it comprises two LR circuits, two operational amplifiers, two counters, etc and it is unable to detect its own failure.
One object of the invention is to provide a circuit comprising the minimum of components and, as already mentioned, capable of signalling its own failure.